The transfer of human force input on a typical crank type device is inefficient due to the crank type device not transferring all the force input to the rotational output. Human input onto crank arms can be in the form of muscle exertion and gravitational forces of the human mass onto crank arms. In the pure form a force is applied to a crank type device in one rotational direction. The only time the force is applied at 100 percent is when the force is applied at a 90 degree angle to the lever position. When the force is applied at a 0 degree angle, 0 percent of the force is applied to rotation. FIG. 1 demonstrates the concept of the force distribution on a bicycle crank in which the force is applied from the top towards the bottom.
As shown in FIG. 1, when the lever position (LP) is directly above the crank (referred to as the zero position), the potential energy (P) in the downward direction is at 100 percent. However, since the lever rotates rather than sliding up and down, the kinetic energy (K) is zero as there is no movement in the downward direction. As the lever position rotates 22.5 degrees counterclockwise, the potential energy of the downward force is at approximately 75% and the kinetic energy is at approximately 25%. As the lever position continues to rotate counterclockwise to 45 degrees, the potential energy decreases to approximately 50 percent while the kinetic energy increases to approximately 50 percent. When the lever position reaches a 90 degree angle relative to the application of force, the potential energy is at 0 percent and the kinetic energy reaches its maximum 100 percent. As rotation continues in the counterclockwise direction the lever position reaches 135 degrees where the potential energy increases to approximately 50 percent and the kinetic energy decreases to approximately 50 percent. When the lever position is at its lowest point at 180 degrees, the kinetic energy reaches its minimum 0 percent. Therefore, a rotating crank assembly does not maximize the available kinetic energy throughout most of its movement.
Rotating crank arms waste a lot of the energy from muscle exertion and gravitational forces because the energy is dissipated into different directions. Due to the inefficiencies of existing cranking devices, there is still a need for efficient cranking devices.